The present invention relates to electrochemical energy storage devices (electrochemical cells). More particularly, the invention relates to a method of fabricating a polymer-cased battery cell having a porous separator.
Due to the increasing demand for battery-powered electronic equipment, there has been a corresponding increase in demand for rechargeable electrochemical cells having high specific energies. In order to meet this demand, various types of rechargeable cells have been developed, including improved aqueous nickel-cadmium batteries, various formulations of aqueous nickel-metal hydride batteries, nonaqueous rechargeable lithium-metal cells and nonaqueous rechargeable lithium-ion cells. While rechargeable lithium-metal cells have high energy densities and specific energies, they have historically suffered from poor cycle life, discharge rate, and safety characteristics, and so have not gained widespread acceptance.
Lithium-ion cells (sometimes referred to as xe2x80x9clithium rocking chair,xe2x80x9d or xe2x80x9clithium intercalationxe2x80x9d cells) are attractive because they preserve much of the high cell-voltage and high specific-energy characteristics of lithium-metal cells. Because of their superior performance characteristics in a number of areas, they quickly gained acceptance in portable electronics applications following their introduction in the early 1990""s. Lithium-ion cells retain their charge considerably longer than comparable nickel-cadmium (NiCad) cells and are significantly smaller, both of which are desirable characteristics since manufacturers seek to make electronic products smaller and portable.
Battery cells are primarily composed of a positive electrode, a negative electrode, and an ion-conducting separator interposed between the two electrodes. Conventional lithium-ion battery cells have typically used as a separator a porous polymer film, such as polyethylene, polypropylene, polytetrafuoroethylene, polystryrene, polyethyleneterphtalate, ethylenepropylene diene monomer (EPDM), nylon and combinations thereof, filled with an electrolyte solution. Also, conventional cells are enclosed in a rigid case, typically made of stainless steel, in order to apply pressure to the cell components to maintain good electrical connections between the components.
In order to reduce the size and weight of battery cells, more recently attempts have been made to develop lithium-ion battery cells which do not require the rigid case in order to maintain good electrical connections between the battery cell""s components. Instead of rigid cell casings, these cells may be packaged in polymer pouches. Various adhesives and binders have been proposed in order to provide sufficient adhesive strength between the components of such polymer-cased cells. Such binders include, for example, polyurethane, polyethylene oxide, polyacrylonitrile, polymethylacrylate, polyacrylamide, polyvinylacetate, polyvinylpyrrolidone, polytetrafluoroethylene, glycol diacrylate, polyvinylidene fluoride (PVDF), and copolymers of the foregoing and combinations thereof.
It is well known that a porous separator enhances the performance of a lithium-ion battery cell by facilitating electrolyte and ion flow between the electrodes. Typical separators used in lithium-ion battery cells are porous polymers, such as polyethylene, polypropylene or mixtures thereof. Previously described methods for fabricating polymer-cased lithium-ion battery cells have involved applying a binder resin solution, such as PVDF, to a porous separator, for example composed of polyethylene, and then adhering and laminating the positive and negative electrodes to the binder-coated separator. Thereafter, the binder resin solvent was evaporated to form the battery cell electrode laminate. Subsequently, the laminate was impregnated with electrolyte solution in a pouch, which was then sealed to complete the cell.
It has previously been noted that PVDF is a particularly useful binder material for polymer-cased battery cells. In addition to providing good lamination between the solid electrochemical components of a battery cell (electrodes and separator), cured PVDF confers rigidity to the battery cell package. The introduction of PVDF as a binder has resulted in improved lithium-ion battery cells, however, there is little reported work on determining optimal selection and use of PVDF materials in electrochemical cells.
It is believed that some conventional cells use PVDF KYNAR 461 as a binder in order to facilitate the assembly of cells due to that PVDF""s low melting temperature which required only a low lamination temperature. However, cell integrity has been found to be insufficient to withstand the high temperature requirements that many the original equipment manufactures (OEMs) require.
Other cells have been made using PVDF copolymers (such as PVDF-HFP) that have low crystallinity, low melting point, and relatively low molecular weight. It is believed that such materials are not the most appropriate ones for optimal structural integrity in a gel-polymer lithium-ion battery.
Thus, processes and materials for facilitating the fabrication and improving the performance of electrochemical cells, including lithium-ion battery cells, would be desirable.
To achieve the foregoing, the present invention provides alternative fabrication methods and compositions for an electrochemical cell. The methods and compositions of the present invention are particularly, though not exclusively, applicable to the manufacture of polymer-cased lithium-ion secondary battery cells. Briefly, the present invention provides an electrochemical cell fabrication process wherein a PVDF-based binder specifically selected for its physical and chemical properties, in particular, its high crystallinity, is coated on a porous separator material to form a porous separator. The high crystallinity PVDF of the binder results in improved cell structure and performance.
In one aspect, the invention provides a method of making an electrochemical cell separator. The method involves contacting a porous separator material with a solution of a binder material, including polyvinylidene fluoride (PVDF) homopolymer having a crystallinity greater than about 50%. The PVDF will also preferably have a molecular weight greater than about 300,000, and a melting point greater than 160xc2x0 C.
In another aspect, the invention provides an electrochemical cell separator. The separator includes a porous separator material and a porous coating of a binder formed on the separator material, the binder including a PVDF homopolymer having a crystallinity greater than about 50%. The PVDF will also preferably have a molecular weight greater than about 300,000, and a melting point greater than 160xc2x0 C.
In other aspects, the invention provides electrochemical cells and methods of their manufacture using separators fabricated in accordance with the present invention.
These and other features and advantages of the present invention are described below with reference to the drawings.